We report measurements on the electrooptic response of thin samples (sim2-5 b.mum) of two antiferroelectric liquid crystals. All the phase transitions in these compounds can be very easily detected using this technique. We have been able to measure such an electrooptic effect for the first time in the antiferroelectric and smectic I* phases of a tolane compound. The response shows a relaxation at high frequencies (sim10 KHz) and is at-tributed to a soft mode which produces an asymmetry in the molecular tilt in successive layers.